The gene encoding for transcription factor 7-like 2 () is the strongest type 2 diabetes mellitus (T2DM) candidate gene discovered to date. The TCF7L2 protein is a key transcriptional effector of the Wnt/β-catenin signaling pathway, which is an important developmental pathway that negatively regulates adipogenesis. However, the precise role that TCF7L2 plays in the development and function of adipocytes remains largely unknown. Using a combination of in vitro approaches, we first show that TCF7L2 protein is increased during adipogenesis in 3T3-L1 cells and primary adipocyte stem cells and that TCF7L2 expression is required for the regulation of Wnt signaling during adipogenesis. Inactivation of TCF7L2 protein by removing the high-mobility group (HMG)-box DNA binding domain in mature adipocytes in vivo leads to whole-body glucose intolerance and hepatic insulin resistance. This phenotype is associated with increased subcutaneous adipose tissue mass, adipocyte hypertrophy, and inflammation. Finally, we demonstrate that mRNA expression is downregulated in humans with impaired glucose tolerance and adipocyte insulin resistance, highlighting the translational potential of these findings. In summary, our data indicate that TCF7L2 has key roles in adipose tissue development and function that may reveal, at least in part, how TCF7L2 contributes to the pathophysiology of T2DM.

Highlights d L-lactate triggers ER Mg 2+ release that promotes mitochondrial Mg 2+ uptake d Mg 2+ is a second messenger for metabolic circuits d Limiting Mrs2-mediated Mg 2+ uptake enhances mitochondrial bioenergetics d Inflammation-induced lactate contributes to organ failure via m Mg 2+ surge

The ability to maintain skeletal muscle mass appears to be impaired in insulin resistant conditions. The present study investigated the effect of lipid induced insulin resistance on the rate of muscle protein synthesis. Seven healthy male volunteers (23 ± 1 y, 24 ± 1 kg/m2) underwent a 7 h intravenous infusion of [ring‐2H5]phenylalanine (0.5 mg/kg/h) on two randomised occasions combined with either 0.9% saline or 10% Intralipid (100 mL/h; Fresenius Kabi, Germany). After a 4 h ‘basal’ period, a 21 g bolus of amino acids (except phenylalanine and tyrosine) was administered in a 440 mL solution nasogastrically, and a 3 h euglycaemic (4.5 mmol/L) hyperinsulinemic (50 mU/m2/min) clamp was commenced (‘fed’ period). Muscle biopsies were obtained from the vastus lateralis at 1.5, 4, and 7 h. Lipid infusion resulted in elevated levels of plasma free fatty acids when compared to saline (P<0.001), which reduced fed glucose disposal by 20% (P<0.01) and pyruvate dehydrogenase complex activation by 50% (P<0.05). Furthermore, whereas mixed muscle fractional synthetic rate increased from the basal to fed period during saline infusion (0.040 ± 0.010 to 0.067 ± 0.013 %/h; P<0.05), it did not respond during lipid infusion (0.048 ± 0.013 to 0.038 ± 0.005 %/h), despite the same circulating insulin and leucine concentrations. Thus, lipid induced insulin resistance results in anabolic resistance to amino acid ingestion in healthy young men.

Insulin resistance is closely related to intramyocellular lipid (IMCL) accumulation, and both are associated with increasing age. It remains to be determined to what extent perturbations in IMCL metabolism are related to the aging process per se. On two separate occasions, whole-body and muscle insulin sensitivity (euglycemichyperinsulinemic clamp with 2-deoxyglucose) and fat utilization during 1 h of exercise at 50% VO 2max ([U-13 C]palmitate infusion combined with electron microscopy of IMCL) were determined in young lean (YL), old lean (OL), and old overweight (OO) males. OL displayed IMCL content and insulin sensitivity comparable with those in YL, whereas OO were markedly insulin resistant and had more than twofold greater IMCL in the subsarcolemmal (SSL) region. Indeed, whereas the plasma free fatty acid R a and R d were twice those of YL in both OL and OO, SSL area only increased during exercise in OO. Thus, skeletal muscle insulin resistance and lipid accumulation often observed in older individuals are likely due to lifestyle factors rather than inherent aging of skeletal muscle as usually reported. However, age per se appears to cause exacerbated adipose tissue lipolysis, suggesting that strategies to reduce muscle lipid delivery and improve adipose tissue function may be warranted in older overweight individuals.The global prevalence of type 2 diabetes is most apparent in older people (1), and it is estimated that the number of people over 65 years of age with diabetes will have increased 4.5-fold by 2050 (2). Gaining mechanistic insight into age-related insulin resistance and strategies to improve insulin sensitivity with age are clearly warranted. Although aging is associated with insulin resistance, age per se does not appear to cause insulin resistance (3-5). Several factors that likely contribute to age-related insulin resistance include increased abdominal adiposity and reduced physical activity (3,4), along with declines in muscle mass (6,7). Of note, intramyocellular lipid (IMCL) accumulates with age, particularly in subsarcolemmal (SSL) regions (8), and has been strongly associated with insulin resistance (9-12). Indeed, SSL lipid accumulation has been linked to the accumulation of metabolites, such as diacylglycerol (DAG) and ceramide, thought by some (13-15), but not others (16), to contribute to impaired insulinstimulated muscle glucose uptake. Nevertheless, it remains contentious as to which factors associated with age influence IMCL accumulation.The accumulation of IMCL and associated metabolites likely results from an imbalance between muscle lipid delivery and oxidation. Indeed, studies have demonstrated reduced free fatty acid (FFA) oxidation in older people compared with young, despite whole-body lipolysis and plasma FFA availability being greater at rest and during exercise at the same absolute and relative intensities (17,18). Linked to this, several studies have suggested that age-related blunting of FFA oxidation and increased IMCL accumulation are a result of reduced

Key points• Carnitine is a substrate for the carnitine palmitoyltransferase 1 enzyme, a rate-limiting step in fatty acid oxidation within skeletal muscle.• Insulin stimulates carnitine transport into skeletal muscle.• A 20% increase in muscle carnitine content, achieved via 12 weeks of twice daily supplementation of a beverage containing 1.36 g of L-carnitine and 80 g of carbohydrate (in order to stimulate insulin-mediated muscle carnitine transport), prevented an 18% increase in body fat mass associated with carbohydrate supplementation alone in healthy young men.• A novel finding of the present study was that this prevention of fat gain was associated with a greater energy expenditure and fat oxidation during low-intensity physical activity, and an adaptive increase in expression of gene networks involved in muscle insulin signalling and fatty acid metabolism.• Implications to health warrant further investigation, particularly in obese individuals who have a reduced reliance on muscle fat oxidation during exercise.Abstract Twelve weeks of daily L-carnitine and carbohydrate feeding in humans increases skeletal muscle total carnitine content, and prevents body mass accrual associated with carbohydrate feeding alone. Here we determined the influence of L-carnitine and carbohydrate feeding on energy metabolism, body fat mass and muscle expression of fuel metabolism genes. Twelve males exercised at 50% maximal oxygen consumption for 30 min once before and once after 12 weeks of twice daily feeding of 80 g carbohydrate (Control, n = 6) or 1.36 g L-carnitine + 80 g carbohydrate (Carnitine, n = 6). Maximal carnitine palmitolytransferase 1 (CPT1) activity remained similar in both groups over 12 weeks. However, whereas muscle total carnitine, long-chain acyl-CoA and whole-body energy expenditure did not change over 12 weeks in Control, they increased in Carnitine by 20%, 200% and 6%, respectively (P < 0.05). Moreover, body mass and whole-body fat mass (dual-energy X-ray absorptiometry) increased over 12 weeks in Control by 1.9 and 1.8 kg, respectively (P < 0.05), but did not change in Carnitine. Seventy-three of 187 genes relating to fuel metabolism were upregulated in Carnitine vs. Control after 12 weeks, with 'insulin signalling' , 'peroxisome proliferator-activated receptor signalling' and 'fatty acid metabolism' as the three most enriched pathways in gene functional analysis. In conclusion, increasing muscle total carnitine in healthy humans can modulate muscle metabolism, energy expenditure and body composition over a prolonged period, which is entirely consistent with a carnitine-mediated increase in muscle long-chain acyl-group translocation via CPT1. Implications to health warrant This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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The sodium-glucose co-transporters (SGLTs) are responsible for the tubular reabsorption of filtered glucose from the kidney into the bloodstream. The inhibition of SGLT2-mediated glucose reabsorption is a novel and highly effective strategy to alleviate hyperglycaemia in patients with type 2 diabetes mellitus (T2DM). However, the effectiveness of SGLT2 inhibitor therapy is diminished due, in part, to a compensatory increase in the maximum reabsorptive capacity (Tm) for glucose in patients with T2DM. We hypothesized that this increase in Tm could be explained by an increase in the tubular expression of SGLT and glucose transporters (GLUT) in these patients. To examine this, we obtained human kidney biopsy specimens from patients with or without T2DM and examined the mRNA expression of SGLTs and GLUTs. The expression of SGLT1 is markedly increased in the kidney of patients with T2DM, and SGLT1 mRNA is highly and significantly correlated with fasting and postprandial plasma glucose and HbA1c. In contrast, our data demonstrate that the levels of SGLT2 and GLUT2 mRNA are downregulated in diabetic patients, but not to a statistically significant level. These important findings are clinically significant and may have implications for the treatment of T2DM using strategies that target SGLT transporters in the kidney.

The tricarboxylic acid (TCA) cycle converts the end products of glycolysis and fatty acid β-oxidation into the reducing equivalents NADH and FADH2. Although mitochondrial matrix uptake of Ca2+ enhances ATP production, it remains unclear whether deprivation of mitochondrial TCA substrates alters mitochondrial Ca2+ flux. We investigated the effect of TCA cycle substrates on MCU-mediated mitochondrial matrix uptake of Ca2+, mitochondrial bioenergetics, and autophagic flux. Inhibition of glycolysis, mitochondrial pyruvate transport, or mitochondrial fatty acid transport triggered expression of the MCU gatekeeper MICU1 but not the MCU core subunit. Knockdown of mitochondrial pyruvate carrier (MPC) isoforms or expression of the dominant negative mutant MPC1R97W resulted in increased MICU1 protein abundance and inhibition of MCU-mediated mitochondrial matrix uptake of Ca2+. We also found that genetic ablation of MPC1 in hepatocytes and mouse embryonic fibroblasts resulted in reduced resting matrix Ca2+, likely because of increased MICU1 expression, but resulted in changes in mitochondrial morphology. TCA cycle substrate–dependent MICU1 expression was mediated by the transcription factor early growth response 1 (EGR1). Blocking mitochondrial pyruvate or fatty acid flux was linked to increased autophagy marker abundance. These studies reveal a mechanism that controls the MCU-mediated Ca2+ flux machinery and that depends on TCA cycle substrate availability. This mechanism generates a metabolic homeostatic circuit that protects cells from bioenergetic crisis and mitochondrial Ca2+ overload during periods of nutrient stress.